With the widespread application of hydrogen as an ecologically clean fuel and a potential substitute for petroleum-based fuels, the safety issues concerning handling of hydrogen on a large scale become a high priority. The ability to monitor hydrogen leakage at production, storage, transmission and usage sites is crucial for its safe handling and use.
The low explosion limits of hydrogen combined with the lack of odor and invisibility of its flame makes the detection of this gas especially difficult and dangerous. Hydrogen has the highest diffusivity of any gas, thus, when it leaks, it can form an explosive mixture with oxygen in the air, which has a lower explosive limit of about 4% H2 in air. The use of odorants for detecting hydrogen leaks by sensing odor, similar to odorizing natural gas by mercaptans, may not be desirable because this could potentially create problems in some hydrogen application areas, such as fuel cells, a liquid hydrogen fuel for Space Shuttle, and the like.
Conventional hydrogen detectors and commercially-available hydrogen sensors fall into five main categories: catalytic combustion, electrochemical, semiconductor-based, thermal conductivity and chemochromic or visual detectors. The existing hydrogen leak detectors and sensors have several drawbacks. First, their use is time consuming and laborious especially when very large areas must be monitored.
Secondly, some of them are not conducive for continuous monitoring, and many exhibit loss of sensitivity in the field due to wind effects, UV radiation from sun, and the like. Many conventional sensors are susceptible to much interference and some of them, for example, catalytic combustion type, are not specific to hydrogen and may produce a false alarm if other combustible gases such as methane, ammonia, carbon monoxide, or propane are present.
Some of the existing formulations involve very active combustion catalysts that may present a fire hazard during a contact with combustible materials. Many hydrogen sensors are rather complicated, bulky devices and, in many cases, may require dedicated personnel to work with the equipment. Among chemochromic type hydrogen detectors some of them are not stable in the environment and could crack, peel, or be washed off by rain. Existing hydrogen sensors have rather limited temperature endurance and cannot operate at high temperatures, for example, above 200° C., because of the potential damage to the system components, which limits their application areas.
Another disadvantage of the existing hydrogen sensors is that a hydrogen leak can be detected only as it is occurring, whereas in some practical systems it is important to know if a leakage has occurred at some point in the past. Lastly, most of the known hydrogen sensors are expensive because they involve costly equipment to produce and operate them or expensive materials to manufacture them, for example, high vacuum systems, fiber optic cables, palladium (Pd) films and the like.
U.S. Pat. No. 6,895,805 to Hoagland teaches a hydrogen gas indicator system having a multi-layered structure and comprising of the layers of hydrogen sensor material such as oxides of vanadium, yttrium, tungsten, molybdenum with the layer thickness of 0.2-10 microns, a catalyst material such as palladium (Pd), platinum (Pt), nickel (Ni), rhodium (Rh) with the layer thickness of 0.5-10 microns and a protective molecular diffusion barrier (e.g., Pd coating or a polymer film). The shortcoming of this invention is that it is rather difficult to manufacture the hydrogen sensor since it is multi-step process, which requires specialized expensive equipment, such as vacuum vapor deposition apparatus, sputtering, electrophoretic devices. Furthermore, the multi-layered structures are cumbersome to manufacture in a controlled manner.
Similarly, gasochromic hydrogen-sensing coating comprising the layers of WO3 and Pt on a glass substrate was described by Witter et al. in Solar Energy Materials and Solar Cells, 84, 305, (2004). The WO3 coating with the thickness of less that 1 micron is further coated with a film of Pt produced by sputtering technique. Exposing the coating to hydrogen resulted in its coloring; the process could be reversed by exposing the coating to air. This indicates that the system is reversible, which implies that hydrogen can be detected only when it is present.
Ping et al. in U.S. Patent Publication No. 2004/0023595 disclosed a thin-film hydrogen sensor consisting of Pd layer coated on the water-doped WO3 layer coated on a substrate. The production of water-doped WO3 films is accomplished by plasma-enhanced chemical vapor deposition method, which requires expensive special equipment. However the major problem with Pd-film coated films is that in the presence of high concentrations of hydrogen, palladium (Pd) hydride may be formed, which would adversely affect the stability of the film and the device, in general. Other shortcomings relate to the fact that the system works only in a reversible mode, and it may not work at temperatures above approximately 100° C., because water will evaporate from-doped WO3 films.
Liu et al. in U.S. Patent Publication No. 2004/0037740 discloses a device for colorimetric hydrogen detection. It consists of a glass substrate, a vanadium oxide layer coated on the glass substrate and a palladium layer coated on the vanadium oxide layer. The disadvantage of the system is that the substrate is limited to a glass, which may significantly limit its application areas. Moreover, the device has a layered structure, which is difficult to manufacture in a controlled manner. Furthermore, the system operates in reversible mode only.
Mendoza et al in U.S. Pat. No. 6,535,658, describes a fiber optic system that employs a chemochromic indicator using layers of tungsten oxide and palladium. The procedure for making hydrogen-sensing material is complicated as it involves six steps. The device itself is also very complex since it includes a light source, an optical fiber, a glass rod and an optoelectronic detector. A similar concept is disclosed by Lee et al in U.S. Pat. No. 6,723,566. The inventors used a double layer gasochromic sensor structure comprising a glass substrate, tungsten-doped nickel oxide layer coated on the glass substrate and a palladium layer coated on the tungsten-nickel oxide layer. The system involves a complex optoelectronic system for monitoring hydrogen.
U.S. Pat. No. 5,849,073 to Sakamoto discloses a pigment for detecting hydrogen gas leakage comprising one of the platinum group metals and one of the compounds of aluminum, silicon, titanium, zinc, zirconium, tin, antimony and cerium. Since the compositions disclosed are typically quite impervious to gas penetration, the method requires very thin coatings, typically a thickness of 2 mils, with relatively high concentrations of active chemochromic compounds. The disadvantage of Sakamoto's formulations is that they are not specific to hydrogen.
Puri et al in U.S. Patent Publication 2004/0115818 teaches an apparatus for detecting a leak of fluid, such as hydrogen, from a vessel having inner and outer walls that includes a chemical material layer adjacent to the outer wall. The chemical material undergoes a chemical reaction with the fluid leaking through the outer wall producing a detectable odor or discoloration of the chemical material layer. The inventors propose to use ethanol solution of carotene in the presence of Raney nickel for detecting hydrogen by color change from yellow to colorless. This method, however, is rather complicated and may not be applicable for a variety of applications, such as detecting H2 leak from joints, fittings, and the like.
Smith and Tracy describe a hydrogen sensor consisting of a hydrogen sensitive chemochromic coating, such as tungsten oxide, at the end of an optical fiber in a Preprint of Papers, Amer. Chem. Soc., Div. Fuel Chem., 2004, 49, 968. A thin catalytic over-layer of palladium acts as a dissociation catalyst forming atomic hydrogen, which subsequently reacts with the WO3. Pd layer coated by 300 nm thick coating of nano-phase titania. The shortcomings of this device are similar to those of multi-layered hydrogen sensors. Moreover, the system operates only in a reversible mode.
A gas-permeable chemochromic composition for hydrogen sensing is disclosed by Bokerman et al in U.S. Patent Publication 2007/0224081. The hydrogen sensor is based on a composition comprising a gas permeable matrix material encapsulating a chemochromic pigment that changes color in the presence of hydrogen. The encapsulating matrix materials are crosslinked polymers, preferably, silicone rubber or silicone resin. The chemochromic pigments are PdO and compounds of Mo and W supported on metal oxides, preferably, TiO2 and Al2O3. The sensor has a layered structure consisting of at least two layers: a top layer comprising a pigment in a silicone matrix disposed on a clear silicone overcoat layer or a support that does not include pigment. Several shortcomings to the disclosed hydrogen sensor are as follows. The sensor is rather cumbersome to prepare since it requires a certain skill in producing a thin layer or a double-layer of the silicone matrix material. Furthermore, the disclosed hydrogen sensor requires a rather expensive material for manufacturing, a moisture curing silicone adhesive/sealant, which will increase the cost of the final product. Thirdly, due to the presence of a polymer matrix, which acts as a diffusion barrier for gases, the kinetics of color change is relatively slow. PdO-based pigment is not specific to hydrogen. Lastly, the presence of the polymer matrix significantly narrows the working temperature range of the hydrogen sensor from about minus 40° C. (below that temperature the polymer matrix hardens) to about plus 250° C. when it starts decomposing.
Thus, an improved low-cost, user-friendly and easy-to-prepare hydrogen detector with high detection specificity and sensitivity, resistance to chemical degradation, operable in a wide range of temperatures from minus 100° C. to plus 500° C., dependable and durable, stable and reproducible, with variable reversibility and inherently safe to operate in any environment is needed. The potential application areas for such a hydrogen detector include aerospace, transportation, hydrogen plants, oil refineries, ammonia plants, hydrogen reformers, fuel cell plants, chemical and analytical laboratories, and the like.
The present invention improves upon and overcomes many of the deficiencies of the prior art.